1. Field of the Invention
This invention relates to traction drive fluids, and more particularly to such traction drive fluids which can be used not only for the driving force transmitting mechanisms but also for hydraulic pressure controlling mechanism and the friction properties controlling mechanisms of the wet clutches of automobiles. The traction drive fluids of the present invention are particularly useful for traction drive type continuously variable transmissions of automobiles.
2. Description of the Prior Art
In the field of industrial machines, traction drive fluids have already been used in traction drive type power transmitting apparatuses which transmit power via oil film. Such traction drive fluids are required to have a high traction coefficient which indicates a power transmission capability.
In recent years, extensive studies and investigations on a traction drive fluid have progressed for its use of the continuously variable transmissions of automobiles. Traction drive fluids to be used for automobiles are expected to be used not only in the power transmitting mechanisms but also in the hydraulic controlling mechanisms and the friction properties controlling mechanisms for the wet clutch.
An automatic transmission fluid (ATF) is known as a lubricating oil used in automobile's transmission hydraulic controlling mechanism and friction properties controlling mechanism of wet clutches. It is a well-known fact that such an ATF is required to have a higher kinematic viscosity at elevated temperatures than a certain level and to be superior in low temperature flowability for performing the role in hydraulic controlling mechanisms. It is also well known that an ATF needs to be blended with additives which have excellent friction properties, particularly anti-shudder properties for fulfilling requirements in performing the role in the friction properties controlling mechanism, particularly the controlling mechanism having in addition thereto slip controlling capabilities.
Therefore, when used in the traction drive type continuous variable transmission of an automobile, a traction drive fluid is required to have driving force transmitting properties which are peculiarly excellent but also properties as a fluid for hydraulic controlling, and friction properties controlling of a wet-type clutch, both of which are required for an ATF.
It is also a well-known fact that a fluid tends to be deteriorated in low-temperature flowability as the traction coefficient increases. Therefore, in the development of a traction drive fluid, it is a major problem how such trade-off relation between traction coefficient and low temperature flowability is deal with.
More specifically, there is a problem that the traction coefficient of a fluid would be reduced if it can achieve the possession of the pour point at -10.degree. C. or below which is required as a traction drive fluid for industrial machines of which usage is limited to indoors or the possession of the low temperature pour point which is required to be also used in the hydraulic controlling mechanisms of traction drive type continuous variable transmissions which have been developed for automobiles.